The present invention relates to expandable screws, and more particularly, to an expandable screw apparatus and method for driving and expanding the same in a structure. In addition, the present invention relates to an assembly for the same.
Screw fixation is commonly used in spinal internal posterior fixation surgeries for various spinal disorders such as spondylolisthesis, trauma, tumors, deformities and other conditions. Many screw fixation systems have been developed and are used increasingly to connect different designs of such screws to structural members such as plates, rods, bars, and the like. In these systems, pedicle screws are often employed, which are cancellous bone screws that can sustain high loads. The strength of the bone-screw interface is crucial, because the interface must be strong enough to withstand the correcting forces applied intraoperatively and stable enough to avoid deformation under physiologic loading after surgery. A poor bone-screw interface can decrease the strength and stability of the construct and cause pseudoarthrosis, loss of correction, and failure of surgeries due to screw loosening or pullout, for instance, in an osteoporotic patient. The bone-screw interface is affected by variables such as bone mineral density (BMD), screw design, insertion technique, and supplementary instruments. Particularly, the BMD has a critical influence on the bone-screw interface. However, host bone quality mostly is beyond a surgeon""s control. Therefore, increasing the strength of the bone-screw interface by focusing on screw design and screw hole preparation continues to be an important aspect of study in screw fixation systems.
Various systems have been implemented for fixating screws into structures such as bone. For instance, simple pedicle screws that did not expand were driven into bone in the past. However, this type of screw did not optimize holding power and therefore the bone-screw interface strength lacked. In addition, supplemental devices have been used to augment the holding strength of fixation devices. For example, polymethylmethacrylate (PMMA) has been employed as an injection cement to increase holding power of fixated screws. However, PMMA may lose its fixative strength over time and can cause neurological injury by extrusion of the cement. Therefore, it has not been used routinely. Also, supplemental devices have included laminar hooks in combination with pedicle screws and/or a washer or staple combined with vertebral screws. Furthermore, other devices such as an anchor have been used in combination with transvertebral screws. However, uses of these supplemental instruments are not ideal as they can require the recruitment of additional bone structures and can significantly increase the complexity of the fixation procedure, or they are not practical for fixation into particular structures, such as bone.
Still a more useful pedicle screw has been developed, wherein the screw is expandable at the driven end upon insertion into a bone structure or the like. However, as this screw is expanded after the formation of bone threads, the bone threads tend to compress thereby compromising the holding condition of the bone.
Accordingly, there remains a need for a means of increasing the strength of the structure-screw interface. Also, there is yet a need for a less complicated and more convenient way of fixating screws to a surface and/or structure.
In accordance with the present invention, the above and other problems were solved by providing an expandable screw apparatus and an expandable screw driving and expanding assembly, as well as a method for inserting and expanding an expandable screw.
In one embodiment of the present invention, an expandable screw apparatus includes an outer screw having a longitudinal hollow core and a plurality of longitudinal expandable slits proximate a distal end, and an inner screw, disposed inside the longitudinal hollow core of the outer screw, having a distal end extended outside the longitudinal hollow core of the outer screw. Further, the expandable screw apparatus includes an end nut, engaged with the distal end of the inner screw, wherein by rotating the outer screw relative to the inner screw while simultaneously holding the inner screw, the end nut is drawn into the hollow core of the outer screw, thereby causing simultaneous expansion of the expandable slits in a transversal direction.
Further in one embodiment of the present invention, a method is provided for fixation of an expandable screw apparatus into a structure. First, an expandable screw apparatus is provided, which includes an outer screw having a longitudinal hollow core, an inner screw, and a end nut. The expandable screw apparatus is driven a length inwards without expanding the screw apparatus. Then the end nut connected to the inner screw can be engaged into one of a plurality of longitudinal slits at a distal end of the outer screw by rotating the inner screw to draw the end nut inwards. After the end nut is engaged, the expandable screw apparatus can continue being driven inwards by simultaneously holding the inner screw in place to prevent the inner screw from rotating, and rotating and driving the outer screw thereby moving the end nut proximally into the longitudinal hollow core of the outer screw towards the proximal end of the inner screw, the end nut includes a portion having a larger diameter than a diameter of the longitudinal hollow core. By simultaneously rotating the outer screw of the expandable screw apparatus and holding the inner screw of the expandable screw apparatus, which prevents the inner screw from rotating, the expandable screw is being driven into a structure, while simultaneously being expanded in an outwardly transversal direction from the longitudinal hollow core at its longitudinal slits.
Yet in one embodiment in a method of fixating an expandable screw, the end nut can be pre-engaged into one of the longitudinal slits at the end of the outer screw, thereby eliminating the need for the step of engaging the end nut as described in the above embodiment.
Still in one embodiment, an assembly for driving and expanding an expandable screw apparatus, includes an outer screw, an inner screw, and an end nut as described in the above embodiment. In addition, the assembly contains a driver mechanism and an elongated stabilizing member. The driver mechanism contains a structural end connectable to a structural head of the outer screw, the driver mechanism also includes a hollow shaft. The elongated stabilizing member includes an end insertable first through the hollow shaft of the driver mechanism towards the proximal end of the outer screw and through the longitudinal hole region of the outer screw. The elongated stabilizing member is connectable to an insertion space of the proximal end cap of the inner screw. Furthermore, the elongated stabilizing member provides rotating means for the end nut to engage the longitudinal slits. Moreover, during dynamic expansion, the elongated stabilizing member provides holding means to prevent the inner screw from rotating.
One advantage of the present invention is that the employment of an expandable screw can increase the holding strength to the structure it is fixed to or therein. In addition, the method of the present invention allows an expandable screw to be inserted such that the strength of the structure-screw interface is improved, and also provides a less complicated way for inserting an expandable screw.